Camshafts in a vehicle engine are often driven by timing chains and a drive sprocket or sprockets (or gears) off the front of the crankshaft, or off the front of an idler/balance shaft in some engine designs. Chain guides, tensioner arms and tensioning devices (which may be hydraulic or spring actuated) are used to maintain chain tension. A respective driven sprocket is attached for rotation with each camshaft and is driven rotationally by a timing chain. The camshafts control the opening and closing motion of engine valves that regulate airflow into and out of engine cylinders. The airflow is created by the upward and downward motion of pistons that is generated by the rotary motion of the crankshaft converted to linear motion by connecting rods.
The timing of the opening and closing of the engine valves in relation to the crankshaft is critical due to a typically low clearance of the pistons to the intake valves when opening and to the exhaust valves when closing. To accommodate this tight clearance, gasoline engines often have valve relief pockets cast or machined into the pistons to provide additional valve to piston clearance. Diesel engines have significantly higher compression ratios, with most of the volume of the combustion chamber in the crown of the piston. A machined or cast valve relief pocket puts a stress concentration in the crown area of the piston. Therefore, to maintain control of the combustion chamber volume and eliminate a piston stress concentration, diesel engines minimize the size of, or do not use valve relief pockets. This requires a lower running clearance between the pistons and valves. Diesel engines must therefore be designed and assembled to attain such a precise clearance.